1. Technical Field
The present invention relates to a method of controlling an output of an ultrasonic speaker that drives ultrasonic transducers with modulated signals obtained by modulating ultrasonic-frequency-band carrier waves with audible-frequency-band sound waves (for example, audio signals), an ultrasonic speaker system, and a display device.
2. Related Art
An ultrasonic speaker is used, for example, to transmit sound information to a specific place because the ultrasonic speaker is highly directional as compared with a typical speaker. For example, an ultrasonic speaker is provided such that when persons approach a work of art in an exhibition hall, such as an art gallery, only limited persons around the work of art may hear explanation on the work of art.
However, it is known that a sound pressure level of a self-demodulated sound within an audible band in an ultrasonic speaker becomes lower as closer to a low band. In addition, the sound pressure level of a self-demodulated sound in an ultrasonic speaker is proportional to an amplitude value (that is, degree of modulation in the case of a modulated wave) of a signal wave. Accordingly, in order to realize the satisfactory sound quality in the ultrasonic speaker, it is necessary to set a degree of modulation in the middle and low frequency band as high as possible so as not to be overmodulated and to set a degree of modulation in the middle and high frequency band to be lower than that in the middle and low frequency band.
A device for adjusting base and treble of a specified audio signal is proposed (refer to JP-A-6-216681). As can be seen from the device, in the case of changing volume in each band by means of a base that changes a frequency characteristic of an audio signal in the middle and low frequency band and a treble that changes a frequency characteristic of the audio signal in the middle and high frequency band, a typical audio device generally has a base adjustment knob and a treble adjustment knob in an amplifier. In the case of a typical audio device, only by adjusting the volume in each band by the use of base and treble, it is possible to sufficiently make an adjustment to proper sound quality and volume with respect to various music sources.
However, when a technique of adjusting the volume in each band by means of the base and the treble is applied to an ultrasonic speaker in the same manner as in the above-described typical audio device, a base adjustment knob and a treble adjustment knob are provided to a modulator so as to change the amplitude of an audio signal in each band. This is the same as changing the degree of modulation in each band, and the same operation as described above can be actually performed in the above device. However, in reality, when listening to audio sources, such as music, through an ultrasonic speaker, the sound pressure level in the middle and low frequency band can be adjusted within only a low range even if the degree of modulation is adjusted corresponding to each frequency band as described above. As a result, the satisfactory quality of sound is not obtained.
Here, an audio signal refers to a signal obtained by mixing signals having various frequency components. When comparing dynamic ranges of signal wave amplitude of, for example, a single sine wave having a frequency of 0.5 kHz and amplitude of 1 V and a mixed wave obtained by mixing a sine wave having a frequency of 0.5 kHz and amplitude of 1 V and a signal wave having a frequency of 2 kHz and amplitude of 1 V, the maximum amplitude of the former single sine wave is 1 V and the maximum amplitude of the latter mixed wave is 2 V.
Here, a case is assumed in which the amplitude of the single sine wave and the amplitude of the mixed wave can be changed, carrier waves (having constant amplitude) are modulated in each of the single sine wave and the mixed wave, and the degree of modulation of each modulated wave is set to be 100%. In this case, assuming that the degree of modulation of a modulated wave in the case of the single sine wave is 100% in a state in which the amplitude of the single sine wave (components corresponding to 0.5 kHz) is 1 V, the degree of modulation of a modulated wave in the case of the mixed wave is 100% in a state in which the amplitude of the mixed wave, that is, the amplitude of components (corresponding to 0.5 kHz) included in the mixed wave is 0.5 V. Accordingly, when the two types of modulated waves are emitted from different ultrasonic transducers (whose performance is the same), the sound pressure level of a self-demodulated sound corresponding to 0.5 kHz is larger in the single sine wave than in the mixed wave.
The above state may also be changed by the phase of each signal wave that is mixed, but the state is considered to be true for most of the cases. Accordingly, in order to set the degree of modulation in the middle and low frequency band to be large in the ultrasonic speaker (in order to increase a sound pressure level in the middle and low frequency band), it is important to divide audio signals into a plurality of frequency bands and to reduce frequency components, which are included in signals within the middle and low frequency band, to the minimum.
In order to realize those described above, it is necessary to make frequency components included in each of the bands smaller than frequency components included in original audio signals by causing the audio signals to pass through a plurality of band pass filters, to modulate carrier waves with signal waves in respective frequency bands, and to drive ultrasonic transducers provided corresponding to the respective frequency bands with modulated waves in the respective frequency bands.
As described above, in order to set the degree of modulation in the middle and low frequency band to be large in the ultrasonic speaker (in order to increase the sound pressure level in the middle and low frequency band), it is important to divide audio signals into a plurality of frequency bands, to make frequency components included in each of the bands smaller than frequency components included in original audio signals, to modulate carrier waves with signal waves in the respective frequency bands, and to drive ultrasonic transducers provided corresponding to the respective frequency bands.
Further, in the case when sound waves are emitted from ultrasonic transducers provided corresponding to respective frequency bands, it is an issue that a large amount of power is consumed as compared with a case in which a single ultrasonic transducer is driven.
Furthermore, it is necessary to adjust the degree of modulation and amplitude of a modulated wave in correspondence with each frequency band. Due to many parameters, this causes a problem in which an adjustment to proper sound quality and volume with respect to various audio signals is not easily made, unlike the device disclosed in JP-A-6-216681.